feat: improve search reliability and add latest dataset update tracking

[elainefan331]

Summary

This PR improves NeuroJSON search behavior and adds persistent sync/update tracking for the landing page.

It includes two search fixes, PostgreSQL-backed statistics snapshots, dataset-level incremental change tracking, a latest-update API, and the new LatestUpdateBoard UI.

What changed

Search

• Fix dataset-level keyword matching when subject filters are active by checking the corresponding dbinfo row.
• Prevent the false “No matching subjects” warning from flashing during slow searches.
• Add an index on iolinks(dbname, dsname, view) to improve file-type filtering performance.

Statistics

• Move landing-page database statistics from the legacy CGI path to PostgreSQL.
• Add stats_history to persist one statistics snapshot per sync run.
• Update the sync pipeline to write dataset, subject, file, and size totals after successful syncs.
• Serve landing-page statistics from the latest successful stats_history snapshot.

Dataset change tracking

• Add dataset_changes to track dataset-level added, updated, and deleted events during incremental syncs.
• Link each change to its corresponding stats_history run.
• Keep firstSync() excluded from change logging.

Latest update API

• Add GET /dbs/updates/latest.
• Return the most recent successful sync that contains actual dataset changes.
• Include:
  • added / updated / deleted dataset counts
  • affected datasets
  • net subject, file, and size deltas based on stats_history snapshots

Landing page

• Add fetchLatestUpdate service / Redux action / slice support.
• Add LatestUpdateBoard to the landing page.
• Display the latest update date, dataset changes, subject/file/size deltas, and affected datasets.
• Refine typography, colors, spacing, and alignment to match the existing NeuroJSON theme.

Testing

• Verified keyword + subject-filter searches.
• Verified slow-search warning behavior.
• Verified incremental dataset add / update / delete tracking.
• Verified dataset_changes rows are linked to the correct sync history.
• Verified the latest-update endpoint response.
• Verified landing-page statistics and latest-update rendering.

Related issue

Closes #145